in which xjt{\displaystyle x_{jt}} is one of the n{\displaystyle n} monetary components of the monetary aggregate Mt{\displaystyle M_{t}}. The summation index implies that all monetary components contribute equally to the money total, and it views all components as dollar for dollar perfect substitutes. It has been argued that such an index does not weigh such components in a way that properly summarizes the services of the quantities of money.

There have been many attempts at weighting monetary components within a simple-sum aggregate. An index can rigorously apply microeconomic- and aggregation-theoretic foundations in the construction of monetary aggregates. That approach to monetary aggregation was derived and advocated by William A. Barnett (1980) and has led to the construction of monetary aggregates based on Diewert's (1976) class of superlative quantity index numbers. The new aggregates are called the Divisia aggregates or Monetary Services Indexes. Salam Fayyad's 1986 PhD dissertation did early research with those aggregates using U.S. data.

Here, the growth rate of the aggregate is the weighted average of the growth rates of the component quantities. The discrete time Divisia weights are defined as the expenditure shares averaged over the two periods of the change

which is the opportunity cost of holding a dollar's worth of the j{\displaystyle j}th asset. In the last equation, rjt{\displaystyle r_{jt}} is the market yield on the j{\displaystyle j}th asset, and Rt{\displaystyle R_{t}} is the yield available on a benchmark asset, held only to carry wealth between different time periods.

In the literature on aggregation and index number theory, the Divisia approach to monetary aggregation, MtD{\displaystyle M_{t}^{D}}, is widely viewed as a viable and theoretically appropriate alternative to the simple-sum approach. See, for example, International Monetary Fund (2008), Macroeconomic Dynamics (2009), and Journal of Econometrics (2011). The simple-sum approach, Mt{\displaystyle M_{t}}, which is still in use by some central banks, adds up imperfect substitutes, such as currency and non-negotiable certificates of deposit, without weights reflecting differences in their contributions to the economy's liquidity. A primary source of theory, applications, and data from the aggregation-theoretic approach to monetary aggregation is the Center for Financial Stability in New York City. More details regarding the Divisia approach to monetary aggregation are provided by Barnett, Fisher, and Serletis (1992), Barnett and Serletis (2000), and Serletis (2007). Divisia Monetary Aggregates are available for the United Kingdom by the Bank of England, for the United States by the Federal Reserve Bank of St. Louis, and for Poland by the National Bank of Poland. Divisia monetary aggregates are maintained for internal use by the European Central Bank, the Bank of Japan, the Bank of Israel, and the International Monetary Fund.